1. Field of the Invention
The present invention relates to a method for producing a group, III-V compound semiconductor, and a semiconductor light emitting device using such a semiconductor and a method for producing the same. More specifically, the present invention relates to a semiconductor light emitting device having an excellent efficiency of utilizing a material and excellent heterostructure interface characteristics and a method for producing the same, and a method for producing a group III-V compound which provides such a semiconductor light emitting device.
2. Description of the Related Art
Conventionally, a light emitting diode device and a laser device employing a GaAs type semiconductor material have been used as a semiconductor light emitting device. The colors (wavelength) of light emitted by these semiconductor light emitting devices are in the range from infrared to red.
However, in the case where the writing of information to an optical disk is performed using laser beams generated from these semiconductor light emitting devices, even if the writing of higher density information is to be performed, the density of information to be written is restricted due to the laser beams having a long wavelength. For this reason, a semiconductor light emitting device (semiconductor laser diode) capable of generating laser beams having a shorter wavelength has been increasingly desired.
Accordingly, a short wavelength light emitting a device has been developed by using a group III-V compound semiconductor material including nitrogen (hereinafter, also referred to as a nitride type semiconductor material). Since the group III-V compound semiconductor material has a wide bandgap of 2 eV or more, a short wavelength light emitting device capable of emitting light in a wide range from orange to ultraviolet can be obtained.
At present, a short wavelength light emitting device is generally produced in the following manner: a structure of a device and the compositions of semiconductor layers are determined so that the device can emit light of an intended wavelength by sequentially depositing the semiconductor layers formed of a mixed crystal of a predetermined metal selected from In, Ga and Al and nitrogen at different compositions; and the semiconductor layers are formed by performing crystal growth based on the predetermined structure of the device and the predetermined compositions of the semiconductor layers so as to produce a light emitting device.
For example, according to a crystal growth method for producing a device mainly composed of GaN which emits blue light, an organometallic such as trimethylgallium (TMGa), trimethylaluminum (TMAl), trimethylindium (TMIn) as a group III material and ammonia (NH.sub.3) as a group V material are used (see Japanese Journal of Applied Physics Vol. 30/No.12A (1991), pp. 1998).
However, in such a case where a semiconductor film is grown using ammonia, the high decomposition temperature of ammonia makes it difficult to lower the film forming temperature (hereinafter, also referred to as growth temperature). Accordingly, crystal growth must be performed at high temperatures, and as a result, the following problems arise.
For example, the device which emits blue light mainly composed of GaN has a structure where an active layer (light emitting layer) is constituted by an In.sub.y Ga.sub.1-y N layer (0.ltoreq.y&lt;1), and cladding layers interposing the active layer are constituted by an Al.sub.x Ga.sub.1-x N layer (0.ltoreq.x.ltoreq.1).
In the production of such a blue light emitting device, in order to grow a compound semiconductor film which is crystal-structurally or optically satisfactory using a group III material and ammonia, an Al.sub.x Ga.sub.1-x N layer (0.ltoreq.x.ltoreq.1) requires a temperature of 1000.degree. C. or more as the growth temperature. On the other hand, an In.sub.y Ga.sub.1-y N layer (0.ltoreq.y&lt;1) requires a temperature of 800.degree. C. or less as the growth temperature in order to suppress evaporation of In atoms (see, for example Journal of Electronic Materials/Vol.21/No.2 (1992), p. 157).
Thus, in the case of the production of a heterostructure of a compound semiconductor film including In and a compound semiconductor film not including In, which is an effective structure as a nitride type semiconductor light emitting device, after one of the compound semiconductor layers is grown, the growth is temporarily interrupted in order to change the growth temperature. For this reason, quality deterioration and/or dislocation are caused by heat during the period required for changing the growth temperature, especially at the interfaces of the heterostructure (i.e., between semiconductor layers having different compositions). As a result, the characteristics of the film deteriorate.
A light emitting device having a semiconductor multilayered structure including a plurality of compound semiconductor layers formed of a quaternary semiconductor material of InAlGaN poses similar problems. The quaternary semiconductor material constituting an active layer should have an energy bandgap different from the quaternary semiconductor material constituting a cladding layer. Accordingly, the active layer is different from the cladding layer in the content of In. As a result, the active layer has a different growth temperature from the cladding layer. Therefore, also in the semiconductor light emitting device using the quaternary semiconductor material of InAlGaN, after a predetermined semiconductor layer is grown, it is necessary to temporarily interrupt the crystal growth of the semiconductor layer to change the growth temperature, as in the case of the device with the heterostructure. For this reason, similarly, the quality is altered and/or dislocation occurs at the interface of the semiconductor layers having different compositions, thus resulting in the deterioration of the characteristics of the film.
Furthermore, regarding the Al.sub.x Ga.sub.1-x N film, since vapor pressure of the film is high at a high temperature more than 1000.degree. C., a large number of molecules constituting the film sublime during crystal growth (e.g., nitrogen atoms go out of a crystal). As a result, structural, optical and electrical characteristics of an obtained film are not sufficient for practical purposes.
Furthermore, since the bonding strength between hydrogen and nitrogen is stronger than the bonding strength between an organic substance and a group III metallic element, ammonia inefficiently decomposes by heat under growth conditions for the nitride semiconductor. Therefore, a sufficient amount of nitrogen atoms cannot be obtained without supplying an extremely large amount of ammonia with respect to a group III material. Accordingly, a supply ratio of a group V material to a group III material (hereinafter, referred to as V/III ratio) is necessarily as high as about 10000. As a result, the crystal growth method using ammonia also presents the problem of a very poor efficiency of utilization of a material.
In order to solve this problem, as a method for growing a group III-V compound semiconductor containing nitrogen at a low temperature, for example a method of employing an amine type material such as tertiary-butylamine as the group V element has been proposed (see Proceedings of 55th Applied Physics Conference 19a-MG-4, I-pp180). It has also been reported that crystal growth is performed using an organometallic compound having group III-nitrogen bonds as a single material (Japanese Laid-Open Patent Publication No. 4-139097 and Chemistry of Materials/Vol. 1/No. 1 (1989) p. 119).
The above-mentioned method of employing an amine type material (e.g., tertiary-butylamine) has an advantage in that crystal growth can be performed at a low growth temperature such as about 800.degree. C. In this method, however, the amine type material cannot be sufficiently decomposed, so that the following drawbacks are presented: (1) carbon atoms contained in the amine type material are incorporated into the obtained semiconductor film; and (2) nitrogen in the amine type material and an organometallic does not sufficiently react with each other, so that droplets of a metal Ga or the like are formed on the surface of the obtained semiconductor film. These impurities (e.g., carbon atoms) or droplets deteriorate the light emitting characteristics of the semiconductor film. Thus, in the case where the semiconductor film is used for, for example a light emitting device or the like, a light emitting device having sufficient light emitting characteristics cannot be obtained.
According to the method of employing an organometallic compound having group III-nitrogen bonds as a single material, the material is not sufficiently decomposed on the surface of the substrate, so that the following problems are presented: (a) carbon atoms contained in the material are incorporated into the obtained semiconductor film; and (b) a growth efficiency of the semiconductor film relative to a material supply is low.
On the other hand, it is described in Appl. Phys. Lett. 59(17)21 Oct. 1991, p. 2124 that in the growth of a GaAs type semiconductor layer, a growth rate of a GaAs type crystal can be raised by using azo-tertiary-butane as a radical source. In this case, since the growth temperature of the GaAs type crystal is lowered, the problem caused by a high crystal growth temperature involved in the nitride type semiconductor (i.e, sublimation of molecules constituting the semiconductor film by heat) is prevented.
However, in the semiconductor multilayered structure in the semiconductor light emitting device using a GaAs type semiconductor material (e.g., a structure including a GaAs layer as an active layer and AlGaAs layers as cladding layers interposing the active layer), the growth temperature for the GaAs layer is substantially the same as the growth temperature for the AlGaAs layers, regardless of the use of azo-tertiary-butane in the production process (crystal growth). Accordingly, the technique described in the Appl. Phys. Lett. 59(17), 21 Oct. 1991, p.2124 does not solve the problem characteristic of the crystal growth of the nitride type semiconductor (i.e., the problem resulting from the difference between the growth temperatures due to different compositions of the semiconductor layers).
Furthermore, this technique does not solve the problem in crystal growth of the nitride type semiconductor that an efficiency of utilization of the material is poor because the V/III ratio is very high.